?
It could be that you simply don't have the root's environment set up after this command. If you do

sudo su -

(note the "-"), then the profile of the root user will be read and the environment (among other things PATH variable) will be set as if you have logged in as root. If you run
sudo su, you keep your environment and get the effective uid of 0, meaning you have the privileges of root user without having root environment set up. sudo su - changes your current directory to the root's home directory, so I use sudo su if I want to execute a couple of commands in the current dir (e.g. change owner of a file) without having to type in long paths.